1. Field of the Invention
This invention relates to a physical quantity detecting device for controlling the position of a movable electrode displaced in response to a predetermined physical quantity such as acceleration, a yaw rate or the like based on electrostatic forces produced by energizing fixed electrodes provided on both sides of the movable electrode as seen in its displacing direction, and detecting the physical quantity from a controlled variable on its position.
2. Description of Related Art
As this type of physical quantity detecting device, an acceleration sensor has heretofore been known which has a detecting element provided with a movable electrode and a pair of fixed electrodes, a capacitance detector for detecting a difference in capacitance between differential capacitors each composed of a movable electrode and respective fixed electrodes, and a pulse width modulator for generating pulse-width-modulated (PWM) signals having duty cycles proportional to the detected differential capacitance, where the output of the pulse width modulator is applied to one of the fixed electrodes as it is and applied to the other electrode through an inverter to alternately energize the pair of fixed electrodes, where the movable electrode is controlled to be held in position by electrostatic forces produced between the respective electrodes and the PWM signals are set as detected outputs.
However, the detecting element employed in such an acceleration sensor has a problem in that since it is fabricated on, for example, a semiconductor substrate by etching, detected sensitivity of the acceleration sensor and offsets of the detected outputs thereof vary due to variations in its fabrication.
On the other hand, for example, Japanese Patent Application Laid-Open Publication No. Hei 3-293565 discloses a device wherein the output of a pulse width modulator is applied to one of fixed electrodes through a first inverter and applied to the other electrode through a second inverter for further inverting the output of the first inverter, and a voltage value of a first reference voltage supply for supplying power to the first and second inverters can be adjusted and the output of a second reference voltage supply whose voltage value can be adjusted is added to the output of a capacitance detector by an adder.
In this device, crest values of PWM signals applied to the fixed electrodes through the first and second inverters, in turn, electrostatic forces produced between the fixed electrodes and a movable electrode upon energizing the fixed electrodes change. Therefore, the detected sensitivity can be adjusted by adjusting the voltage value of the first reference voltage supply. Further, errors produced in the output of the capacitance detector according to an offset of a detecting element can be canceled by adjusting the voltage value of the second reference voltage supply.
However, when the voltage value of each reference voltage supply is adjusted as in this type of device, some resistors constituting the reference voltage supply are normally formed as thin film resistors and the thin film resistors are trimmed to vary their resistance values, thereby varying the voltage value. Alternatively, each reference voltage supply must be composed of a D/A converter. Accordingly, such a prior art device has the following problems.
Namely, the trimming of the thin film resistors will cause a problem in that since the resistance values are changed by burning off a pattern with lasers, adjustment working requires time and effort, and the resistance values can be changed only in one direction to increase them, and thereby when voltage-value adjustments end in failure, the voltage value cannot be readjusted, thus creating a difficulty in its adjustment.
On the other hand, another problem arises in that since the number of elements increases as the D/A converter is improved in its adjustment accuracy where the D/A converter is used as the reference voltage supply, the elements are increased in size when one attempts to ensure sufficient adjustment accuracy, and particularly when the entire configuration is implemented in a semiconductor integrated circuit, the cost of a chip increases.
In order to allow adjustments to the detected sensitivity, the use of the aforementioned construction for adjusting the voltages requires that the fixed electrodes are alternately energized to control their energization ratio. Accordingly, the above-described problems cannot be avoided.